Flexible Light-Emitting Diodes Based On All-Inorganic Perovskite Quantum Dot

With the increasing demand for smart and wearable electronic devices,portable,solution-machinable and low-cost flexible perovskite light-emitting diodes（Pe LEDs）have attracted the attention of researchers.Perovskite quantum dots（QDs）have been widely used as emission layers of LED because of their tunable emission wavelength,high photoluminescence quantum yield（PLQY）,high color purity and low cost.The external quantum efficiency（EQE）of more than 20%has been achieved in green and red Pe LED based on rigid glass substrates,and the EQE of blue devices has reached18%.However,the low endurance temperature and poor wettability of flexible substrates,as well as the mechanical stability of the devices,greatly limit the development of flexible Pe LEDs.Therefore,to improve the performance of flexible Pe LEDs,systematic studies have been performed.The device structure is optimized to achieve carrier balance,the luminescent layer is optimized to improve PLQY and stability,and the flexible substrate is optimized to improve mechanical stability.The main contents and results of this paper are as follows.（1）Firstly,a Pe LED device based on a mixed hole transport layer（HTL）was prepared by co evaporation method,which has the advantages of good film quality,low roughness and accurate proportion control in comparison to solution-processed method.By optimizing the proportion of the 1,1-bis-（4-bis（4-tolyl）-aminophenyl）cyclohexene（TAPC）and 4,4’,4’’-tris（carbazol-9-yl）triphenylamine（TCTA）in HTL,the hole injection barrier is reduced and the hole transport ability is effectively enhanced.As a result,the Pe LEDs achieved more balanced carrier transport and enhanced device performances with a peak EQE of 10.7%and luminance of 3308 cd m^-2.（2）Next,the internal quantum efficiency（IQE）of the device is improved by optimizing the perovskite emission layer.Small molecules thiooctanoic acid（TA）were added to the synthesis of CsPb I₃QDs to passivate the surface defects.TA has two functional groups:disulfide bond（S-S）and carboxyl（-COOH）.After heating,the dynamic disulfide bond will exist in the form of cross-linking to form Poly（TA）.The carboxyl group can interact with the uncoordinated Pb of perovskite,thus passivating perovskite and improving the PLQY and stability of,which can passivate defects,improve the PLQY.and stability of QDs.Then Pe LEDs based on TA passivation were prepared.Compared with the pristine CsPb I₃QD LED,the optimized Pe LED not only has better stability,but also has a maximum EQE of 13.8%and a maximum brightness of 1836 cd m^-2.Then polyethylene terephthalate（PET）was used as substrates to prepare flexible Pe LEDs.The brightness of the TA passivated Pe LED decreased to 73%after 1000 bending times at a radius of 3 mm,which was higher than 62%of the comparison device.These results show that Poly（TA）can not only passivate perovskite,improve efficiency and stability,but also improve the mechanical stability of perovskite films.（3）Based on this,cheap and environmentally friendly printing paper was used to replace the original rigid glass substrate and flexible Pe LED was fabricated.Due to the high surface roughness,opacity,high porosity and hygroscopic nature of paper,which cannot be directly used in device fabrication,there have been few studies on paper-based LED so far.In order to overcome these obstacles,we propose to use polymethyl methacrylate（PMMA）as a modification layer to reduce the surface roughness of the printing paper,prevent solvent penetration in the spin coating process and protect the device from water and oxygen.Then,to avoid the opacity of the paper,a top emitting device structure was designed and the external coupling efficiency was enhanced by microcavity effects,enabling the first realization of a Pe LED based on a paper substrate.Consequently,the device based on printing paper/PMMA substrates achieved a peak EQE of 8.5%and luminance of 1330 cd m^-2.After 100 bending times,the device maintained 75%of its initial luminance,which is hopefully applied to disposable portable displays.（4）Finally,to further reduce the effect of roughness on the device performance,we choose a smoother and pastable label paper as the flexible substrate to prepare a high-efficiency flexible Pe LED which is comparable to the rigid devices.In order to explore the main reasons for the decrease of device efficiency when transferred from rigid substrate to flexible substrate,Pe LED based on rigid glass and flexible polyethylene terephthalate（PET）substrates were prepared as comparison devices.Due to the low roughness and good wettability of PMMA modified label paper,each functional layer has excellent moldability.As a result,the performance of the device based on label paper/PMMA substrate has almost no degradation relative to the glass-based device with a record EQE of 14.3%and luminance of 3615 cd m^-2.In addition,due to the lower flexural modulus of label paper than PET substrate,the label paper/PMMA based device exhibits better flexibility.After 1000 bending of times with radius of 3 mm,the device maintained 90%of the initial luminance.We believe that label paper/PMMA based Pe LEDs has great research value in application of portable,cheap,disposable electronic devices,such as electronic identity cards,billboards and labels etc.The innovative points of this thesis are as follows:（1）Facing to the problem of carrier imbalance in devices,TAPC with high hole mobility and appropriate energy levels is doped into TCTA hole transport layer by coevaporation method to enhance hole injection and balance carrier transport.The efficiency of the device has increased from 7.5%to 10.7%.（2）To address the issues of low efficiency and poor stability of the perovskite emission layer,TA was introduced to passivate perovskite QDs.The EQE of rigid Pe LED increased from 7.1%to 13.8%,and the device life T₅₀has been increased from 24 minutes to 75 minutes.The bending stability of flexible Pe LEDs was improved,with the luminance drop reduced from 38%to 27%after 1000 bends.（3）In order to find a flexible substrate with high thermal stability,flexibility,and low cost and environmental frendly,we modified printing paper with PMMA and prepared paper-based Pe LEDs for the first time.And achieved a maximum luminance of 1330 cd/m²and a maximum EQE of 8.5%.The device maintained 75%of its original luminance after 100 bends at a radius of 3 mm.（4）To investigate the reasons for the decrease in device efficiency when transferring from a rigid substrate to a flexible substrate,devices based on rigid glass,flexible PET and label paper was prepared.The effects of roughness,wettability and bending modulus on device performance are also explored.The highest device efficiency of the flexible perovskite QD LED was achieved at 14.3%,as well as excellent bending stability,and the luminance of the device maintained 90%of the initial intensity after bending 1000 times at a radius of 3 mm.